EGR System Optimization for Light-Duty Gasoline Compression Ignition (GCI) Engine 2021-01-0515
Increasingly stringent exhaust and CO2 emissions regulations are driving advancements in combustion and after-treatment technologies in the passenger vehicle sector. One major challenge is to achieve low emissions over the full operating map as required by Real Driving Emissions (RDE) legislation. Gasoline Compression Ignition (GCI), an advanced combustion concept, has shown potential to increase fuel efficiency and reduce emissions. GCI harnesses gasoline’s low reactivity for longer ignition delay, thus promoting partially premixed air-fuel mixture for efficient combustion. To maintain low engine-out NOx over the load range, high Exhaust Gas Recirculation (EGR) is required that consequently elevates boost pressure requirements. To meet the high boost and EGR demands, while minimizing pumping losses require air-system optimization.
This work presents a detailed investigation of EGR system optimization for a prototype 2.6L, four-cylinder, Light-Duty (LD) Gasoline Compression Ignition (GCI) engine using RON92 gasoline at a geometric compression ratio (CR) of 17. Several alternative EGR configurations were evaluated, including conventional layouts such as high-pressure EGR (HP-EGR) loop, low-pressure EGR (LP-EGR) loop, dual-loop EGR (DL-EGR) and a novel EGR configuration. A GT-Power based 1-D engine model was used, to quantify and compared the EGR configurations candidates over both steady-state and transient engine operations. Time-to-torque (TTT) and EGR delivery time were compared.
The HP-EGR alone configuration appeared unfit, by causing inefficient turbocharging, Whereas, the LP-EGR only improved complemented turbocharger performance at the expense of high pumping losses at high engine speeds. A dual-loop EGR system offered a more optimized engine system performance.
A customized (novel) EGR configuration, demonstrated performance benefits equivalent to a dual-loop EGR system without the complexity and packaging issues of DL-EGR system. For steady-state engine operation, the system exhibited pumping benefit in the range of 20 to 90 kPa at engine speed over 2500 RPM. For idle to 2500 RPM full load transient engine condition, the novel EGR configuration enabled approximately two times faster EGR delivery, but, also caused a noticeable delay in TTT, compared to a conventional LP-EGR configuration. Using an integrated starter generator (ISG) assist, the load response lag fully disappeared and the engine system demonstrated almost instantaneous torque. The novel EGR system not only provided the best system performance benefits but also offered superior trade-offs for cost, complexity and integration.